The present invention relates to a method for producing an antenna on a substrate, in particular in plastic substrate, for combination with a chip or a chip module to produce a transponder. In addition, the invention relates to a device with such transponder.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
A transponder is a device for wireless communication, which receives incoming signals and responds automatically. The chip or chip module may be an RFID (Radio Frequency Identification) chip or an RFID chip module. RFID transponders are typically placed on plastic substrates and are further processed as so-called RFID inlays (e.g., RFID units) and the like in contactless cards, e-passports, smart labels and the like. The RFID transponder is hereby used, for example, to identify persons and objects.
In RFID applications relating to people, the RFID chips are provided with special controllers and larger data memories as well as security functions, for example for storing biometric data or for encryption of data for data transmission. These types of RFID chips therefore have a larger size than, for example, RFID chips which are only used to identify products. In the latter, a so-called EPC (Electronic Product Code) number is basically sufficient. Moreover, less memory is required in this case, reducing the size of the silicon chips.
The present description generally uses the term “chip”. This term designates, unless explicitly mentioned otherwise, both silicon chips without a housing (“bare dice”) and chip modules, wherein the chip modules may include both chips with metal substrates (metal lead frames) as well as chips with polymer substrates (so-called “straps” or “interposer”). The term “chip” indicates generally an electronic component having at least one integrated electronic circuit.
Different process and fabrication techniques are used to produce RFID transponder inlays for different applications.
For example, low-cost RFID transponder inlays are produced using the so-called flip chip process. An RFID chip is hereby mounted directly on an antenna arranged on a substrate, wherein the antenna can be made of different materials, for example copper, aluminum or conducting silver paste. The antenna and the chip, which has so-called bump terminals, are typically connected using a conductive adhesive. These adhesives are mostly anisotropic, epoxy-based adhesives (so-called ACP-adhesives) or a “hot melt” adhesive. For hardening the adhesives, the substrates must be designed to withstand temperatures reaching 200° C. for a duration of several seconds, which limits or makes it impossible the use of many substrate materials.
As an alternative to the flip chip method, RFID chip modules can also be connected to antennae arranged on the substrate by other methods. Depending on the materials used for the chip modules, the antenna substrates and the antenna metallizations, the RFID chip module and the antenna are electrically connected with an adhesive, by soldering, welding, or mechanical techniques, such as crimping, clinching, etc.
The antennae are typically made of electrically conducting materials, in particular metals, conductive pastes or conductive inks. For applications in the high-frequency (HF) range—which includes the standard frequency of 13.56 MHz used with RFID—the reading properties are predominantly determined by the ohmic resistance of the antenna. The cross-section of the electrical conductors of the windings in conjunction with the specific resistance of the applied material has a direct effect. For this reason, the antennae used in the present example of an RFID transponder are frequently foil antennae or wire antennae, whereby the foil antennae are preferably made of copper or aluminum and the wire antennae are preferably made of copper wire. The antennae are always applied on plastic substrates. The substrate material is normally PET (polyethylene terephthalate), PEN (polyethylene naphthalate), polyimide (PI, Kaptone®), PVC (polyvinylchloride) or PC (polycarbonate).
UHF antennae for RFID applications with reading ranges of up to several meters operate in the frequency range of about 860-960 MHz and are configured as a single-layer electric dipole, i.e., for example on one side of a foil substrate. The physical thickness is several micrometers.
HF antennae for RFID applications are more particularly employed with security applications due to the required limited writing and reading range (no more than 10 cm). HF antennae typically operate at 13.56 MHz. They are constructed as a coil which in an actual application exchanges data with a second coil in the reading device without making physical contact. The coil is therefore constructed on one side of a plastic substrate material. To allow connection of both ends of the coil to the RFID chip (e.g., a security controller chip), one end of the coil must be routed across the windings of the coil to the vicinity of the other coil end. This is done in different ways, depending on the employed antenna fabrication technique. For example, the wires of wire antennae are coated with an insulating layer. The wire can then be routed to the other wire end across the windings placed on the substrate, without causing a short circuit. In the flatter etching-based fabrication process, the antennae structures are made of a copper foil layer that was previously laminated across an area, similar to the fabrication of etched printed circuit boards. To bring one coil end into the vicinity of the other coil end, a so-called “bridge” is printed across the coil windings. An insulation layer must be printed underneath the “bridge” to prevent a short circuit. After etching, a residue from the adhesive used to laminate the copper foil typically remains on the antenna substrate. The possibility for laminating additional layers of the same material is then limited. This applies particularly to the expensive polycarbonate substrate materials required for secure applications.
For single-layer HF antennae fabricated by a printing method, the bridge technique is produced, as with the etching process, by forming an insulation layer between “bridge” and antenna windings. The wire antenna technique has dominated high-quality applications because these include additional material layers produced in the lamination process with the RFID inlay. The device is constructed conventionally in a single plane, i.e., the HF antenna coil is applied on one side of the plastic substrate and connected with the RFID chip. The side-by-side wires therefore result, by taking into account the employed wire thickness, a coil with a minimum coil width (about 4 to 6 windings, depending on the characteristic properties of the RFID chip) and a minimum thickness of the RFID inlay.
In all the aforementioned methods, the antennae are applied on the substrate separately from the RFID chip. The antenna and the RFID chip are electrically connected in a separate process step.
Disadvantageously, the aforementioned method requires different fabrication, assembly and connection techniques, depending on the employed substrate and antenna materials. More and more frequently, these techniques reach physical limits, when increased productivity is required, for example with respect to temperature loading and hardening duration with adhesives, etc. The stringent quality requirements with respect to reliability and product lifetime, for example during fabrication of RF transponders for identification of persons or contactless applications in financial transactions, therefore significantly limit the techniques and materials that can be used.
It would therefore be desirable and advantageous to provide an improved method for producing an antenna on a substrate, which obviates prior art shortcomings and is able to specifically provide a simple and universal technique for fabricating different types of antennae with a chip or a chip module for an RFID transponder.